<!DOCTYPE html>
<html>
	<head>
		<meta charset="utf-8">
		<title>typeof，类型转换</title>
	</head>
	<body>
	<script type="text/javascript">
		document.write(typeof(a)+"---")
		
		document.write(typeof(undefined)+"<<<<<---输出是string=====>>>>-")
		document.write(typeof(typeof(a)))
	</script>
	</body>
</html>
